Conventional data storing devices store data in files on a storage, and, upon request from a client, read out data from the storage to transmit the read-out data to the client. For data which is frequently accessed, high-speed responses are achieved by locating the data in a DRAM. However, this scheme has a problem that a high-capacity DRAM is required to achieve a high-speed response with a large storage capacity. In addition, in the event of a failure of the storage, there is another problem that responsiveness to a client is largely deteriorated or a server stops functioning.